Why exactly would you want to do this? The macbook you're talking about already comes with 1gb of ram. Also its DDR2 667mhz which is way faster than DDR 333. It might work but it certainly wouldn't be ideal.
tl;dr: Strikes me as a bit of an odd idea

Why exactly would you want to do this? The macbook you're talking about already comes with 1gb of ram. Also its DDR2 667mhz which is way faster than DDR 333. It might work but it certainly wouldn't be ideal.
tl;dr: Strikes me as a bit of an odd idea

Click to expand...

It comes with 2x512MB ram. If I swap 1x512MB with my 1GB card, it would have 1.5GB total, would this be better or worse than 1GB? Taking into account the speed... would it even work?

Ok, found this on the Crucial website, might be useful for someone else:

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest module or system component (the "lowest common denominator" effect).

One thing to keep in mind is that the memory does need to be the same type  for example, SDRAM cannot be mixed with DDR, and DDR cannot be mixed with DDR2. We recommend that you use the Crucial Memory Advisor or System Scanner tools to find the right memory for your computer.

So an iBook uses DDR ram, MacBook uses DDR2 ram. These won't work together. Problem solved. I will need to buy ram if I want it. How much ram does a MacBook need, probs 1.5GB is a decent amount?

As all of the reasons mentioned by other members, and the fat that they are different Voltages. You never mix voltages of RAM. Can be nasty, have never seen the results first hand, but have heard you can frazzle the mb

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